The present invention relates to communications methods and apparatus, and more particularly, to spread spectrum communications methods and apparatus. Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990's. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
Several types of access techniques are conventionally used to provide wireless services to users of wireless systems. Traditional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels, wherein discrete frequency bands serve as channels over which cellular terminals communicate with cellular base stations. Typically, these bands are reused in geographically separated cells in order to increase system capacity. Modern digital wireless systems typically utilize different multiple access techniques such as time division multiple access (TDMA) and/or code division multiple access (CDMA) to provide increased spectral efficiency. In TDMA systems, such as those conforming to the GSM or IS-136 standards, carriers are divided into sequential time slots that are assigned to multiple channels such that a plurality of channels may be multiplexed on a single carrier. CDMA systems, such as those conforming to the IS-95 standard, achieve increased channel capacity by using “spread spectrum” techniques wherein a channel is defined by modulating a data-modulated carrier signal by a unique spreading code, i.e., a code that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates.
Conventional spread-spectrum CDMA communications systems commonly use so-called “direct sequence” spread spectrum modulation (“DS-CDMA”). In direct sequence modulation, a data-modulated carrier is directly modulated by a spreading code or sequence before being amplified by a power amplifier and transmitted over a communications medium, e.g., an air interface. The spreading code typically includes a sequence of “chips” occurring at a chip rate that typically is much higher than the bit rate of the data being transmitted.
In a typical DS-CDMA system, data streams from different users are subjected to various signal processing steps, such as error correction coding or interleaving, and spread using a combination of a user specific spreading code and a group-specific scrambling code. The coded data streams from the users are then combined, subjected to carrier modulation and transmitted as a composite signal in a communications medium.
A RAKE receiver structure is commonly used to recover information corresponding to one of the user data streams in a DS-CDMA system. In a typical RAKE receiver, a received composite signal is typically correlated with a particular spreading sequence assigned to the receiver. Correlations are performed at a plurality of different times (e.g., delays) to produce a plurality of time-offset correlations. The correlations are then combined in a weighted fashion, i.e., respective correlations are multiplied by respective weighting factors and then summed to produce a decision statistic.
The performance of CDMA systems generally is limited by interference among different user signals. Spreading/despreading provides a degree of interference suppression, but the number of users is generally limited by interference. Conventional RAKE reception techniques generally treat interference as uncorrelated (white) noise. Thus, conventional RAKE receivers typically used the complex conjugates of channel coefficients estimated by a channel estimator as weighting factors. Such an approach may yield less than desirable results in CDMA systems, because the passing of interfering signals through the dispersive medium generally introduces correlation into the noise at the receiver.